Reactive multilayer structures for ease of processing and enhanced ductility

ABSTRACT

In accordance with the invention, a reactive multilayer structure comprises alternating layers of materials that exothermically react by a self-propagating reduction/oxidation reaction or by a self-propagating reduction/formation reaction. This combination of a reduction reaction and either an oxidation or formation reaction can lead to ductile reaction products and is frequently accompanied by the generation of large amounts of heat. As compared with conventional multilayer foils, the new multilayer structures are easier to fabricate, easier to handle, and produce more reliable bonds.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication Serial No. 60/201,292 filed by T. P. Weihs et al. on May 2,2000 and entitled “Reactive Multilayer Foils”. It is related to U.S.patent application Ser. No. ______ filed by M. E. Reiss et al.concurrently herewith and entitled “Method of Making Reactive MultilayerFoil and Resulting Product” and U.S. patent application Ser. No. ______filed by T. P. Weihs et al. concurrently herewith and entitled“Freestanding Reactive Multilayer Foils”. These three relatedapplications are incorporated herein by reference.

GOVERNMENT INTEREST

[0002] This invention was made with government support under NSF GrantNos. DMR-9702546 and DMR-9632526, and The Army Research Lab/AdvancedMaterials Characterization Program through Award No. 019620047. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

[0003] This invention relates to reactive multilayer structures, and, inparticular, to reactive multilayer structures that can be easilyprocessed to produce ductile reaction products.

BACKGROUND OF THE INVENTION

[0004] Reactive multilayer coatings are useful in a wide variety ofapplications requiring the generation of intense, controlled amounts ofheat in a planar region. Such structures conventionally comprise asuccession of substrate-supported coatings that, upon appropriateexcitation, undergo a self-propagating exothermic chemical reaction thatspreads across the area covered by the layers. While we will describethese reactive coatings primarily as sources of heat for welding,soldering or brazing, they can also be used in other applicationsrequiring controlled local generation of heat such as propulsion andignition.

[0005] Many methods of bonding require a heat source. The heat sourcemay be external or internal to the structure to be joined. An externalsource is typically a furnace that heats the entire unit to be bonded,including the bodies (bulk materials) to be joined and the joiningmaterial. An external heat source often presents problems because thebulk materials can be sensitive to the high temperatures required forjoining. The bulk materials can also be damaged in cooling from hightemperatures due to mismatches in thermal contraction.

[0006] Internal heat sources often take the form of reactive powder.Reactive powders are typically mixtures of metals or compounds thatreact exothermically. Such powders, developed in the early 1960s,fostered bonding by Self-Propagating, High-Temperature Synthesis (SHS).However, the energy released and its diffusion is often difficult tocontrol in SHS reactions. As a result, bonding by powders may beunreliable or insufficient.

[0007] Reactive multilayer structures, which were subsequentlydeveloped, reduced the problems associated with reactive powder bonding.These structures are typically comprised of thin coatings that undergoexothermic reactions. See, for example, T. P. Weihs, Handbook of ThinFilm Process Technology, Part B, Section F.7, edited by D. A. Glockerand S. I. Shah (IOP Publishing, 1998); U.S. Pat. No. 5,538,795 issued toBarbee, Jr. et al. on Jul. 23, 1996; and U.S. Pat. No. 5,381,944 to D.M. Makowiecki et al. on Jan. 17, 1995. As compared to reactive powders,reactive multilayer structures permit exothermic reactions with morecontrollable and consistent heat generation. The basic driving forcebehind such reactions is a reduction in atomic bond energy. When theseries of reactive layers is ignited, the distinct layers mixatomically, generating heat locally. This heat ignites adjacent regionsof the structure, thereby permitting the reaction to travel the entirelength of the structure until all the material is reacted.

[0008] The individual layer thickness in the foils defines the averagediffusion distance that is required for materials to mix in theseexothermic reactions. An exothermic reaction in a multilayer foil canself-propagate far more easily and far faster at room temperature thanthe same reaction in a powder compact because the layers are many ordersof magnitude smaller than the powders. Individual layer thicknessestypically range from 1-1000 nm while typical powder diameters range from10 to 100 μm. Consequently, reaction velocities in foils typically rangefrom 1-30 m/s while reaction in powders range from 0.01 to 0.1 m/s. Anadditional advantage for multilayer foils is that the thicknesses oftheir individual layers are far more uniform, consistent, andcontrollable than diameters of corresponding powders. Thus, reactionproperties are more easily controlled and modified. Lastly, whilereactive foils are fully dense and free of contaminants at interfacesbetween its reactants, reactive powder compacts are rarely fully denseand often contain many contaminants at reactant/reactant interfaces due,for example, to oxide coatings on the particles. Both the lack of fulldensity and the presence of contaminants can limit reaction kinetics andvelocities compared to reactive foils.

[0009] While a clear improvement over powders, reactive multilayerstructures encountered their own difficulties. For example, whenattached to a substrate, the reactive foils often debond or delaminatefrom their substrates upon reaction. This debonding is caused byinherent reactive foil densification during reaction and by non-uniformthermal expansion on heating and contraction during cooling. In the caseof joining, it significantly weakens the bonding joint. Moresignificantly, most reactive coatings react to produce a brittleintermetallic compound, which can be detrimental at the center of ajoint, lowering its fracture toughness and causing it to behave in abrittle fashion when deformed. Consequently, internal or externalstresses can cause catastrophic mechanical failure of the joint.

[0010] To date, most research and development of self-propagatingreactions in foils has been directed primarily to formation reactionswherein two or more elements (A/B) mix and react to form a compoundproduct (AB_(x)). While such reactions may produce large heats ofreaction, many difficulties are encountered in fabricating and using therequisite foils. The reactants are typically expensive or are brittle,hard to deposit and difficult to use. Many of the foils are subject tounwanted ignition. Moreover, many of the reactions produce brittle finalproducts.

[0011] These difficulties can be illustrated by the problems with foilshaving B, C or Si layers. Table 1, which lists pertinent conventionalformation reactions, shows that many of these reactions combine atransition metal such as Ti, Zr, Hf, V, Nb, Ta, Ni, Pd, or Pt with alight element such as B, C, Si, or Al. It also shows that the borides,carbides, and silicides generally have higher heats than the aluminides.(The two exceptions are very expensive due to the use of Pd and Pt, andtherefore have very limited commercial potential.) Thus reactive foilswith high heats of reaction generally employ reactions that formborides, carbides, or silicides. TABLE 1 Thermodynamic Data forFormation Reactions that Can Self-propagate in Reactive Foils at RoomTemperature Heat of Adiabatic Reaction Reaction Phase of Reaction(kJ/mol) Temperature (° C.) Reaction Product Ti + 2B to TiB₂ −108  2920Liquid Zr + 2B to ZrB₂ −108  3000 Liquid H + 2B to HfB₂ −110  3370Liquid V + 2B to VB₂ −68 2297 Solid Nb + 2B to NbB₂ −72 2282 Solid Ta +2B to TaB₂ −63 2400 solid Ti + C to TiC −93 3067 liquid Zr + C to ZrC−104  3417 liquid Hf + C to HfC −105  3830 liquid V + C to VC −50 1957Solid Nb + C to NbC −69 2698 Solid Ta + C to TaC −72 2831 Solid 5Ti +3Si to Ti₅Si₃ −72 2120 liquid 5Zr + 3Si to Zr₅Si₃ −72 2250 liquid 5Hf +3Si to Hf₅Si₃ −70 2200 liquid 5V + 3Si to V₅Si₃ −58 1519 solid 5Nb + 3Sito Nb₅Si₃ −57 2060 solid 5Ta + 3Si to Ta₅Si₃ −42 1547 solid Ti + Al toTiAl −36 1227 solid Zr + Al to ZrAl −45 1480 liquid Hf + Al to HfAl −46Ni + Al to NiAl −59 1639 liquid Pd + Al to PdAl −92 2380 liquid Pt + Alto PtAl −100  2800 liquid

[0012] Unfortunately, reactive foils with B, C or Si are difficult tofabricate and use. As compared with aluminum, for example, foils with B,C or Si are more likely to delaminate or fracture during vapordeposition. When deposited at the relatively low temperatures requiredfor making reactive multilayer foils, B, C and Si deposit in anamorphous state. Consequently the deposited layers are subject toconsiderable growth stresses. Thus, multilayer foils with amorphouslayers of B, C and Si have a higher driving force to delaminate. Inaddition, multilayer foils with amorphous layers of B, C or Si are moresusceptible to fracture, cracking, and delaminating than foils with Allayers.

[0013] An additional difficulty in fabricating foils with B, C, or Si isthat these materials sputter deposit at very slow rates, far slower thanAl. Since sputter deposition is a preferred method of fabricatingreactive multilayer foils, slow sputter rates are a severe limitation onthe eventual commercialization of these foils.

[0014] Reactive foils that contain B, C, or Si, also tend to be brittleand unstable. The amorphous layers of B, C or Si in these foils have alower fracture toughness than the alternative Al layers, so themultilayer foils will be more susceptible to fracture and crackingduring handling. This susceptibility makes cutting and patterning thefoils difficult and raises the likelihood of unwanted ignition duringhandling due to fracture or cracking. In addition, since transitionelements diffuse rapidly into amorphous layers, faster than into Al at asimilar temperature, foils based on B, C, or Si will also have lowerthresholds for ignition, which also makes them more susceptible tounwanted ignition.

[0015] Lastly, when any of the above formation reactions are completed,the final reaction product is brittle at room temperature. Thus, futurehandling or use of this product, whether in a joint, a propellant, or acombustion reaction, can be degraded. In the particular case of joining,the presence of a brittle boride, carbide, silicide, or aluminide at theinterface between the two components is bound to lower the fracturestrength, fracture resistance, and fatigue resistance of the joint.Accordingly, there is a need for new reactive multilayer foils that canbe easily processed and handled and can be easily used to produceductile, reliable bonding.

SUMMARY OF THE INVENTION

[0016] In accordance with the invention, a reactive multilayer structurecomprises alternating layers of materials that exothermically react by aself-propagating reduction/oxidation reaction or by a self-propagatingreduction/formation reaction. This combination of a reduction reactionand either an oxidation or formation reaction can lead to ductilereaction products and is frequently accompanied by the generation oflarge amounts of heat. As compared with conventional multilayer foils,the new multilayer structures are easier to fabricate, easier to handle,and produce more reliable bonds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The nature, advantages, and various additional features of theinvention can be seen by consideration of the various illustrativeembodiments now to be described in detail in connection with theaccompanying drawings. In the drawings:

[0018]FIG. 1 is a schematic diagram of a reactive multilayer structurein accordance with the invention;

[0019]FIG. 2 is a schematic diagram of a reactive foil with particlecomposite geometry in accordance with the invention; and

[0020]FIG. 3 illustrates bonding using the ductile metal reactionproduct of a multilayer structure as a joining material.

[0021] It is to be understood that these drawings are for purposes ofillustrating the concepts of the invention and, except for graphicalillustrations, are not to scale.

DETAILED DESCRIPTION

[0022] In accordance with a preferred embodiment of the invention, areactive multilayer structure (generically referred to herein as a“foil”) is provided as a local heat source in a variety of applicationssuch as a process for joining materials together. As illustrated in FIG.1, the reactive foil (14) with a layered structure is made up ofalternating layers 16 and 18. The foil contains two materials, which intheir simplest form consist of an element α and an oxide or compoundβΓ_(x), where α, β and Γ can designate any element and x can be aninteger or a fraction. The foil will react by the element (α) reducingthe initial oxide or compound (βΓ_(x)) and forming a more stable oxideor compound αΓ_(y) and the element β. This combination of a reductionand either an oxidation or a formation reaction leads to the release ofheat. Examples include reactions wherein a reactive element like Al (orSi, Ti, Zr, or Hf) reduces an oxide with a low heat of formation (e.g.,Fe₂O₃, CuO, or ZnO) and forms a metal (Fe, Cu, or Zn) plus an oxide,Al₂O₃ (or SiO₂, TiO₂, ZrO₂, or HfO₂) that has a very high heat offormation. Examples also include a reactive element(s) such as Ti, (orZr and Hf) that reduces a compound with a low heat of formation, such asNiB, and then subsequently forms a metal (Ni) plus a compound (TiB₂, orZrB₂ or HfB₂ which has a high heat of formation.

[0023]FIG. 2 illustrates an alternate form of a reactive multilayerstructure which we will call a composite particle foil 50 wherein one ofthe reactive materials is in the form of particles 52 (e.g. spheres,disks or fibers). The other reactive material can be in the form oflayers establishing a matrix 54 for the particles. The structure 50 canuse the same materials and the same reactions as the foil 14 of FIG. 1.

[0024] The materials (α/βΓ_(x)) used in the reactive structures (14, 50)are preferably chemically distinct. In one preferred embodiment, layers16, 18 (52/54) alternate between Al and an oxide with a low heat offormation (e.g., Fe₂O₃, CuO, or ZnO). In another embodiment, layers 16,18 (52,54) alternate between a reactive early transition element such asTi, Zr, or Hf and a boride, silicide, or carbide compound with a lowheat of formation such as (NiB, FeB, FeSi, Cu₃Si, Ni₃C, Fe₃C). In yetanother embodiment, layers 16, 18 (52,54) alternate between Pt, Pd oralloys of these elements and an aluminide compound with a low heat offormation such as (CuAl₂, TiAl₃) In another preferred embodiment, theinitial compounds comprise metallic elements that are ductile such asFe, Cu, or Ni, so that the final product consists of this ductile metaland a hard compound. Preferably, the pairs of materials α, βΓ_(x), arechosen so that the reactions form stable compounds with large negativeheats of reaction and high adiabatic reaction temperatures. Thesereactions will self-propagate in a manner similar to the formationreactions described in T. P. Weihs, “Self-Propagating Reactions inMultilayer Materials,” Handbook of Thin Film Process Technology (1997),which is incorporated herein by reference in its entirety.

[0025] When a multilayer structure 14, 50 is exposed to a stimulus(e.g., a spark or energy pulse) neighboring atoms from the two materialsmix. The change in chemical bonding caused by this mixing results in areduction in atomic bond energy, thus generating heat in an exothermicchemical reaction. This chemical bonding occurs as layers with α-αbonds(i.e., layer 16, 52) and layers with β-Γ bonds (i.e., layer 18, 54) areexchanged for α-Γ and β-β bonds, thereby reducing the chemical energystored in the foil, and generating heat. As FIGS. 1 and 2 furtherillustrate, the heat that is generated diffuses through foil 14, 50 (ina direction from reacted section 30 through reaction zone 32 tounreacted section 34) and initiates additional mixing of the reactants.As a result, a self-sustaining/self propagating reaction is producedthrough the structure 14, 50. With sufficiently large and rapid heatgeneration, the reaction propagates across the entire structure 14, 50at velocities that can exceed 10 m/s. As the reaction does not requireadditional atoms from the surrounding environment (as would, forexample, oxygen in the case of combustion), the reaction makes foils 14or 50 self-contained sources of energy capable of emitting bursts ofheat and light, rapidly reaching temperatures above 1400 K and a localheating rate reaching as high as 10⁹ K/s. This energy is particularlyuseful in applications such as propulsion, joining, and ignitionrequiring production of heat rapidly and locally.

[0026] When a reaction propagates across a multilayer structure 14, 50as illustrated in FIGS. 1 and 2, the maximum temperature of the reactionis typically located at the trailing edge of the reaction zone 32. Thismay be considered the final temperature of reaction, which can bedetermined using the heat of reaction (ΔH_(rx)), the heat lost to theenvironment (ΔH_(env)), the average heat capacity of the sample (C_(p)),and the mass of the product (M). Another factor is whether or the notreaction temperature exceeds the melting point of the final product. Ifthe melting point is exceeded, then some heat is absorbed in the statetransformation from solid to liquid of at least part of the product.With reduction/oxidation and reduction/formation reactions very oftenthe metallic component in the product can melt due to the high reactiontemperatures, while the stable compound may not. The final temperatureof reaction may be determined using the following formulas (where T_(o)is the initial temperature, ΔH_(mm) is the enthalpy of melting of thefinal metallic phase, T_(m) is the melting temperature of the finalmetallic phase in the product, ΔH_(mc) is the enthalpy of melting of thefinal compound phase, and T_(mc) is the melting temperature of the finalcompound phase in the product),

T _(f) =T _(o)−(ΔH _(rx) +ΔH _(env))/(C _(p) M)

[0027] If no melting of final product occurs;

T_(f)=T_(mm)

[0028] If the metallic phase in the product melts only partially;

T _(f) =T _(o)(ΔH _(rx) +ΔH _(env) +ΔH _(mm))/(C _(p) M)

[0029] If the metallic phase in the product completely melts.

[0030] T_(f)=T_(mc)

[0031] If the compound phase in the product melts only partially; and

T _(f) =T _(o)(ΔH _(rx) +ΔH _(env) +ΔH _(mm) +ΔH _(mc))/(C _(p) M)

[0032] If the metallic and compound phases in the product meltcompletely.

[0033] Intricately related to the heat of the foil reaction is thevelocity of the propagation of the reaction along the length of foil 14,50. The speed at which the reaction propagates depends, in particular,on how rapidly the atoms diffuse normal to their layering or particles(FIG. 1 or 2) and how rapidly heat is conducted along the length of foil14, 50. However, now at least three elements are involved in thereaction α, β, and Γ compared to simple formation reactions that caninvolve only two elements. But, only one of the elements must diffuse tocomplete the reduction/oxidation or reduction/formation reactions. Inmost cases the diffusion of O, Si, B, or C between the layers (orparticles and matrix) will control the rate of the reaction. Thepropagation velocity is a strong function of the foil's multilayerthickness or average particle thickness. As the thickness of individuallayers 16, 18 (or particles 54) decreases, the diffusion distances aresmaller and atoms can mix more rapidly. Heat is released at a higherrate, and, therefore, the reaction travels faster through the foilstructure. Reactive foils typically have diffusion distances that rangefrom 1-1000 nm while reactive powder compacts typically have diffusiondistances that range from 10-100 μm. Hence, reaction rates and reactionvelocities are many times faster in foils than in powder compacts.

[0034] In accordance with a preferred embodiment of the invention,reactive multilayer foils 14, 50 may be fabricated by physical vapordeposition (PVD), chemical vapor deposition, electrochemical methods,electroless methods, mechanical methods, or some combination of thesemethods. A magnetron sputtering technique, for example, may be used todeposit the materials α/βΓ_(x) on a substrate (shown in FIG. 1 in dashedoutline form as layer 35) as alternating layers 16, 18. Substrate 35 maybe rotated over two isolated sputter guns in a manner well known in theart to effectuate the layering of materials α/βΓ_(x) into alternatinglayers 16, 18.

[0035] The vapor streams from the two sputter guns or the two electronbeam hearths are isolated from one another during deposition of areactive multilayer foils 14. This isolation reduces intermixing andunwanted reaction of the elements being deposited. It is important toisolate the two vapor streams from one another to prevent loss of theenergy of the reaction during deposition.

[0036] Substrate 35 is shown in dashed outline form to indicate that itis a removable layer that facilitates fabrication of the reactive foil14 as a freestanding foil. Substrate 35 may be any substrate (e.g., Si,glass, or other underlayer) having the characteristics of providingsufficient adhesion so as to keep the foil layers on the substrateduring deposition, but not too adhesive to prevent the foil from beingremoved from the substrate following deposition.

[0037] In accordance with a preferred embodiment, an additional wettinglayer (e.g., tin) may be used as an interface layer between the firstlayer of foil (16 or 18) and the substrate 35 to provide the necessaryadhesive. When no wetting layer is employed, selection of theappropriate material αor βΓ_(x) as the first layer deposited on thesubstrate will ensure that the necessary adhesive requirements are met.When a reactive foil using Al/Cu₂O as materials α/βΓ_(x), is to befabricated, for example, without a wetting layer, the exemplary reactivefoil would be deposited on a substrate such as Si with the first layerbeing Al deposited on the substrate. Al is preferably selected as thefirst layer in such case because Al will sufficiently adhere to Siduring depositing, but will allow peeling off of the substrate after thefoil is formed.

[0038] A fabricated foil 14 may have hundreds to thousands ofalternating layers 16 and 18 stacked on one another. Individual layers16 and 18 preferably have a thickness ranging from 1-1000 nm. In apreferred embodiment, the total thickness of foil 14 may range from 10μm to 1 m.

[0039] Another preferred method of fabricating is to deposit material ina codeposition geometry. Using this method, both material sources aredirected onto one substrate and the atomic fluxes from each materialsource are shuttered to deposit the alternate layers 16 and 18. Again,care must be taken to isolate the two physically distinct atomic fluxesfrom each other.

[0040] In accordance with a preferred embodiment, the degree of atomicintermixing of materials α/βΓ_(x) that may occur during depositionshould be minimized. This may be accomplished by depositing themultilayers onto cooled substrates, particularly when multilayers 16 and18 are sputter deposited. To the extent that some degree of intermixingis unavoidable, a relatively thin (as compared to the alternatingunreacted layers) region of pre-reacted material 20 will be formed. Sucha pre-reacted region 20, nevertheless, is helpful in that it serves toprevent further and spontaneous reaction in foil 14.

[0041] As illustrated in FIGS. 1 and 2, the reactive foil 14 or 50 canhave a layered or particle composite geometry. While a layered geometrywill typically result from vapor deposition of a reactive foil, anotherpreferred embodiment of this invention is the mechanical formation ofα/βΓ_(x) reactive foils. In this method, sheets of α and βΓ_(x) arestacked, inserted in a removable protective jacket, and then deformedinto a multilayer sheet, as by swaging and rolling. The jacket is thenremoved. This mechanical processing can result in either a layered orparticle composite geometry and generally is less expensive than vapordeposition. For further detail concerning mechanical processing, seeU.S. application Ser. No. ______, filed by M. Reiss et al. concurrentlyherewith and entitled “Method of Making Reactive Multilayer Foil andResulting Product” which is incorporated here by reference.

[0042] Reactive foils in accordance with the invention may be adaptedfor use in a variety of applications. In one preferred application, thefoils may be used to ignite another reaction that releases a signal,more heat, or a gas, as in a combustion or propulsion application. Inthis case, a freestanding foil can be inserted into a metastablematerial to be ignited, with all sides of the foil being covered by themetastable material.

[0043]FIG. 3 illustrates another preferred application of the inventionwherein the reactive structures react to form a ductile compositeproduct 40 that contains particles 42 or layers of a hard oxide orcompound in a matrix 41 of a ductile metal. The ductile metal 41 can bea simple element such as Cu, Ni, or Fe or it could be a ductile alloy oftwo or more elements. In a preferred embodiment, these reactivestructures can be used in the joining of two bodies or components(43,44). In these applications, the reactive multilayer is positionedbetween the two bodies (43, 44) to be joined, the bodies are pressedagainst the reactive multilayer, and the latter is ignited. The ductilemetallic product 41 resulting from the self-propagating reaction canserve as a solder or braze that flows and wets the surfaces of thebodies, and consequently forms a strong joint. Thus, reactivemultilayers described in the present invention essentially enable abraze-free room-temperature joining process. This process providessignificant advantages over known reactive joining methods which requirebraze of solder material to be deposited on, or positioned next to thefree-standing foil and/or on the surfaces of the components.

[0044] In another embodiment of the invention, the foils are fabricatedusing inexpensive materials such as Al and CuO, Fe₂O₃ or ZnO. Thesematerials are less expensive than many of the elements used to fabricatereactive foils with very exothermic formation reactions (as opposed toreduction/oxidation or reduction/formation reactions) such as Ti, Zr,Hf, or Nb.

[0045] Preferred embodiments of the invention are useable asfreestanding reactive foils 14 with increased total thickness. The totalthickness of such a reactive foil depends upon the thickness and numberof the elemental layers (e.g., 16 and 18) utilized to form the foils.Foils that are less than 10 μm are very hard to handle as they tend tocurl up on themselves. Foils on the order of 100 μm are stiff, and thus,easily handled. Thicker foils also minimize quenching. In joiningapplications, for example, using reactive foils, there is a criticalbalance between the rate at which the foil generates heat and the rateat which that heat is conducted into the surrounding braze layers andthe joint to be formed. If heat is conducted away faster than it isgenerated, the reaction will be quenched and the joint cannot be formed.The thicker foils make it harder to quench the reaction because there isa larger volume generating heat and the same surface area through whichheat is lost.

[0046] Thicker foils can be utilized with reaction temperatures that arelower, generally leading to more stable foils. Foils with high formationreaction temperatures (as opposed to reduction/oxidation orreduction/formation temperature) are generally unstable and brittle andtherefore are dangerous and difficult to use. Brittle foils, forexample, will crack easily leading to local hot spots (through therelease of elastic strain energy and friction) that ignite the foil.Cutting such brittle foils (e.g., for specific joint sizes) is verydifficult to do as they are more likely to crack into unusable pieces orignite during the cutting process.

[0047] In accordance with a preferred embodiment, the thicker reactivefoils are on the order of 10 μm to 1 cm. Although a number of differentsystems may be employed to create the thick freestanding reactive foils,a unique process in selecting the fabrication conditions isadvantageous. In accordance with a preferred embodiment, for example,deposition conditions such as sputter gas and substrate temperature areadvantageously chosen so that stresses remain sufficiently low in thefilms of the foil as they are grown in the system. Since the stress inthe film times its thickness determines with the driving force fordelamination, the product of stress and thickness should be kept below1000 N/m. Stresses often arise in the films during the fabricationprocess. As the films grow thicker, they are more likely to peel offtheir substrates or crack their substrates than thinner films, therebyruining the final foil production. By characterizing the stresses on thefilms and selecting conditions to minimize the stresses, the fabricationprocess can be completed without the peeling off (or cracking) of thesubstrate.

[0048] Utilizing one or more embodiments of the invention, a number ofdifferent applications can now be performed more effectively andefficiently. For example, freestanding reactive foils can beincorporated directly into solid propellants, enabling the uniform andcomplete combustion of components within the foil with extremely largereleases of heat. Alternatively, a number of materials can now be joinedmore efficiently. Semiconductor devices may be bonded to circuit boardsor other structures, using reduction/oxidation reactions where the finalmetallic product serves as a braze or solder material to join thecomponents.

EXAMPLES

[0049] The invention may now be more clearly understood by considerationof the following examples:

Example 1

[0050] Al/CuO/Cu Composites

[0051] Electrodeposition of Cu from standard cupric sulphate solution isalternated, with electrodeposition of CuO (or Cu₂O) from a 3 M copperlactate, 0.4 M cupric sulphate solution (pH>10 by addition of NaOH). Thealternating layers can be fabricated either by moving the substrate fromone bath to another or by draining and refilling the same bath withdifferent solution. Rotation of the substrate during deposition and thesuspension of a large volume percent of aluminum particles in the eitheror both solutions enables the aluminum particles to be incorporated inthe electrodeposited matrix.

[0052] Instead of an electrodeposited multilayer structure, the pH ofthe copper lactate solution can be modified such that Cu and CuO aredeposited simultaneously. The oxygen content can be controlled byoptimizing the pH, current density, and electrolyte concentrations.

[0053] The aluminum particles can be 100 micron diameter down to 100 nmdiameter. Cold rolling of the electrodeposited foils will create pancakestructures with much smaller average diffusion lengths. Swaging willcreate oblong particles which can then be roll flattened for evenfurther reduction in diffusion distances.

[0054] It is to be understood that the above-described embodiments areillustrative of only some of the many possible specific embodiments,which can represent applications of the principles of the invention.Numerous and varied other arrangements can be made by those skilled inthe art without departing from the spirit and scope of the invention.

What is claimed is:
 1. In a reactive multilayer structure comprisingalternating layers of materials that react exothermically to produce oneor more reaction products, each of the layers having a thickness in therange 1-10,000 nanometers and the multilayer structure having a totalthickness in the range 10 micrometer to 1 centimeter, the improvementwherein: the materials of respective alternating layers react by aself-propagating reduction/oxidation reaction or a self-propagatingreduction/formation reaction.
 2. The improved reactive structure ofclaim 1 wherein one or more of the alternating layers comprises materialin particle form.
 3. The improved reactive structure of claim 1 whereineach pair of alternating layers comprises a first layer including amaterial α in elemental form and a second layer including a compound ofthe form βΓ_(x), the layers reacting exothermically to produce reactionproducts of the form β and αΓ_(y), where α, β, Γ are elements and x, yare integers or fractions, and where the initial compound, βΓ_(x), has alower heat of formation than the final compound, αΓ_(y).
 4. The improvedreactive structure of claim 3 where the compound is an oxide.
 5. Theimproved reactive structure of claim 3 where the compound is selectedfrom the group consisting of Fe₂O₃, CuO, ZnO, and NiB.
 6. The improvedreactive structure of claim 3 where the element is Al and the compoundis selected from the group consisting of Fe₂O₃, CuO, and ZnO.
 7. Theimproved reactive structure of claim 3 where the element is a transitionelement and the initial compound is a boride, silicide or carbide. 8.The improved reactive structure of claim 7 where the transition elementis selected from the group consisting of Ti, Zr, and Hf and the compoundis selected from the group consisting of NiB, FeB, FeSi, Cu₃Si, Ni₃C andFe₃C.
 9. The improved reactive structure of claim 3 where the element isPt or Pd and the initial compound is an aluminide.
 10. The improvedreactive structure of claim 9 where the aluminide is CuAl₂ or TiAl₃. 11.The improved reactive structure of claim 3 where the element is a metal.12. The improved reactive structure of claim 11 where the metal is Fe,Cu or Ni.
 13. The process of bonding two bodies comprising the steps of:disposing between the two bodies an improved reactive structure inaccordance with claim 1 ; pressing the bodies against the reactivestructure; and igniting the reactive structure.
 14. The method of claim13 wherein the reactive structure produces a ductile reaction productthat can serve as a solder or braze material for bonding the two bodies.15. The method of claim 14 wherein the ductile reaction productcomprises metal.